这句话当中when 引导的是定语从句,修饰前面的one evening.
when 除了可以引导定语从句之外还可引导名词性从句：
1 主语从句
when he will be back is still unknown.
2 宾语从句
we don't know when he will be back.
3 表语从句
the question is whenhe will be back.
当然when 所引导的最常见的还是状语从句
when he came back,he found his house broken into.
注意 when 可以引导以上各种从句,但其本身在从句中做状语!